[PATCH] Model sqrtsd as a binary operation with one source operand tied to the destination (PR14221)

[Michael Kuperstein]

Sorry I'm looking at this so late - missed the sqrtss version of it.

The problem I see with this is that it impacts correct code that doesn't want a destructive update, and can use the flexibility we currently have.
E.g, for this:

  declare <2 x double> @llvm.x86.sse2.sqrt.sd(<2 x double>)
  
  define void @sqrtsd(<2 x double> %a, double* %p0, double* %p1) nounwind uwtable ssp {
    %res = tail call <2 x double> @llvm.x86.sse2.sqrt.sd(<2 x double> %a) nounwind 
    %new = extractelement <2 x double> %res, i32 0
    store double %new, double* %p0
    %orig = extractelement <2 x double> %a, i32 0
    store double %orig, double* %p1
    ret void
  }

We now get:

  movapd  (%rcx), %xmm0
  sqrtsd  %xmm0, %xmm1
  movlpd  %xmm1, (%rdx)
  movlpd  %xmm0, (%r8)

And with the patch, we'll get:

  movapd  (%rcx), %xmm0
  movapd  %xmm0, %xmm1
  sqrtsd  %xmm1, %xmm1
  movlpd  %xmm1, (%rdx)
  movlpd  %xmm0, (%r8)

Do you think we can fix the correctness issue without introducing extra copies?


http://reviews.llvm.org/D6885

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